1. Field of the Invention
The invention relates generally to wireless communications, and more particularly to a systems and methods for dimensioning a wireless communication system.
2. Background
In wireless communication systems, radio system engineers must be able to determine the number of base stations required to support a given number of users in a given area. This process, for purposes of this specification and the claims that follow, is referred to as dimensioning the wireless communication system. In a pure circuit switched system, this is not a problem; however, when packet data is incorporated, it becomes difficult to determine the number of base stations required. This is because packet data capability allows a complex mix of subscriber applications such as web access, e-mail, and multimedia applications. These applications greatly increase the number of variables that effect capacity in wireless communication systems. For example, new variables include burstiness of packets, packet size, delay constraints, and delay variations. Unfortunately, the traditional packet data statistics used to track these variables do not correlate with the statistics used to track variables that effect circuit-switched applications.
In particular, the Erlang model traditionally used to measure capacity for voice-centric systems is not applicable to packet data applications. Simplistically, the Erlang model uses the average number of incoming calls per second, the average length of a call in seconds, and the number of lines available to take the calls. If these three variables are known, Erlang""s formula can be applied to determine the percentage of incoming calls for which no line will be available, i.e., the percentage of calls that will be blocked or delayed.
Erlang""s equation has the form:                               E          ⁡                      (                          t              ,              c                        )                          =                                                            t                c                                            c                !                                      ⁡                          [                                                                    ∑                                          xe2x80x83                                                                                                  i                      =                      0                                        ⁢                                          xe2x80x83                                                                            c                    ⁢                                          xe2x80x83                                                                      ⁢                                                      t                    i                                                        i                    !                                                              ]                                            -            1                                              (        1        )            
where t=the number of incoming calls;
and c=the number of lines.
The traffic (t) is often expressed in the dimensionless unit xe2x80x9cErlang.xe2x80x9d Traffic in Erlang is a number that signifies the expected number of lines, or circuits, that would be occupied if there were no blocking. It is found by multiplying the total expected number of calls per time unit, i.e., average incoming calls per second, with the average length per call. The left hand side of equation (1) provides the proportion of calls that are blocked. Therefore, if (t) and (c) were known, the number of blocked calls can be determined.
In actuality, modern systems require a much more complex model than provided by equation (1), but the basic principle is still the same. therefore, radio systems engineers look at the traffic (t), number of circuit (c) available, and the number of blocked calls deemed acceptable, when planning how many base stations are required and where to position them. Because of the complexity of the systems, the traffic (t) is determined based on complex traffic models that account for issues such as the radio resource management scheme and the effect of user mobility on the traffic volume per call. The number of blocked calls is referred to as the grade-of-service (GOS). By specifying the GOS and determining the traffic (t) in accordance with the appropriate traffic model, the radio systems engineer can determine the number of circuits (c) required. GOS is typically applied to the busy hour, which is a sliding 60-minute period during which the maximum total traffic load in a given 24-hour period occurs. Packet data applications, however, do not use GOS. Instead, packet data applications are characterized by quality-of-service (QOS), which is measured by bit-error-ratio (BER). Thus, packet data applications look at the number of bit errors as opposed to the number of blocked calls. Traditional models do not provide a method for equating GOS requirements in circuit switched applications with QOS requirements in packet data applications.
In accordance with an aspect of the invention, systems and methods for dimensioning a wireless communication system that supports both circuit switched and packet data applications are provided as a way to equate GOS and QOS requirements. One feature of this aspect is that engineers may better determine an optimal number of base stations required to support a given number of users in a given service area of the wireless communication system.
In one embodiment, a method of dimensioning a wireless communication system configured to support a plurality of applications is provided, in which, for each application, a percentage of subscribers of the wireless communication system that will use the application is estimated. A subscriber profile for the application is then determined based at least in part on the estimated percentage of subscribers that will use the application. A busy hour traffic per subscriber ratio is then determined for the application based at least in part on the subscriber profile for the application.
The method may further comprise determining a total busy hour traffic per subscriber ratio for the wireless communication system from the busy hour traffic per subscriber ratios for each application. A total traffic capacity is then estimated for the wireless communication system. The number of subscribers the wireless communication system will support is then calculated by dividing the estimated total traffic capacity by the total busy hour traffic per subscriber ratio.
Other aspects, advantages and novel features of the invention will become apparent from the following Detailed Description of Preferred Embodiments, when considered in conjunction with the accompanying drawings.